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                <h4 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h4><p>ArrayList是我们开发中非常常用的数据存储容器之一，其底层是<strong>动态数组</strong>实现的，我们可以在集合中存储<strong>任意类型</strong>的数据，ArrayList是<strong>线程不安全</strong>的，非常适合用于对元素进行<strong>查找</strong>，效率非常高。</p>
<h4 id="线程安全性"><a href="#线程安全性" class="headerlink" title="线程安全性"></a>线程安全性</h4><p>对ArrayList的操作一般分为两个步骤，<strong>改变位置(size)和操作元素(e)</strong>。所以这个过程在<strong>多线程</strong>的环境下是<strong>不能保证具有原子性</strong>的，因此ArrayList在多线程的环境下是线程不安全的。</p>
<h4 id="源码分析"><a href="#源码分析" class="headerlink" title="源码分析"></a>源码分析</h4><h5 id="1-属性分析"><a href="#1-属性分析" class="headerlink" title="1.属性分析"></a>1.属性分析</h5><pre class="line-numbers language-java"><code class="language-java"><span class="token comment" spellcheck="true">/**
 * 默认初始化容量
 */</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> DEFAULT_CAPACITY <span class="token operator">=</span> <span class="token number">10</span><span class="token punctuation">;</span>

<span class="token comment" spellcheck="true">/**
 * 如果自定义容量为0，则会默认用它来初始化ArrayList。或者用于空数组替换。
 */</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> Object<span class="token punctuation">[</span><span class="token punctuation">]</span> EMPTY_ELEMENTDATA <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token comment" spellcheck="true">/**
 * 如果没有自定义容量，则会使用它来初始化ArrayList。或者用于空数组比对。
 */</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> Object<span class="token punctuation">[</span><span class="token punctuation">]</span> DEFAULTCAPACITY_EMPTY_ELEMENTDATA <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token comment" spellcheck="true">/**
 * 这就是ArrayList底层用到的数组
 * 非私有，以简化嵌套类访问
 * transient 在已经实现序列化的类中，不允许某变量序列化
 */</span>
<span class="token keyword">transient</span> Object<span class="token punctuation">[</span><span class="token punctuation">]</span> elementData<span class="token punctuation">;</span>

<span class="token comment" spellcheck="true">/**
 * 实际ArrayList集合大小
 */</span>
<span class="token keyword">private</span> <span class="token keyword">int</span> size<span class="token punctuation">;</span>

<span class="token comment" spellcheck="true">/**
 * 可分配的最大容量
 */</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> MAX_ARRAY_SIZE <span class="token operator">=</span> Integer<span class="token punctuation">.</span>MAX_VALUE <span class="token operator">-</span> <span class="token number">8</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<hr>
<h6 id="扩展：什么是序列化"><a href="#扩展：什么是序列化" class="headerlink" title="扩展：什么是序列化"></a>扩展：什么是序列化</h6><p>序列化是指：将对象转换成以字节序列的形式来表示，以便用于<strong>持久化和网络传输</strong>。</p>
<p>实现方法：实现<strong>Serializable接口</strong>。</p>
<p>然后用的时候拿出来进行<strong>反序列化</strong>即可又变成Java对象。</p>
<hr>
<h6 id="transient关键字解析"><a href="#transient关键字解析" class="headerlink" title="transient关键字解析"></a>transient关键字解析</h6><blockquote>
<p>Java中<code>transient</code>关键字的作用，简单地说，就是让某些被修饰的成员属性变量不被序列化。</p>
</blockquote>
<p>有了<code>transient</code>关键字声明，则这个变量不会参与序列化操作，即使所在类实现了Serializable接口，反序列化后该变量为空值。</p>
<blockquote>
<p>问题：ArrayList中数组声明：<code>transient Object[] elementData;</code>，事实上我们使用ArrayList在网络传输用的很正常，并没有出现空值啊？</p>
</blockquote>
<p>原来：<code>ArrayList</code>在序列化的时候会调用<code>writeObject()</code>方法，将<code>size</code>和<code>element</code>写入<code>ObjectOutputStream</code>；反序列化时调用<code>readObject()</code>，从<code>ObjectInputStream</code>获取<code>size</code>和<code>element</code>，再恢复到<code>elementData</code>。</p>
<blockquote>
<p>问题：那为什么不直接用elementData来序列化，而采用上诉的方式来实现序列化呢？</p>
</blockquote>
<p>原因在于<code>elementData</code>是一个缓存数组，它通常会预留一些容量，等容量不足时再扩充容量，那么有些空间可能就没有实际存储元素，采用上诉的方式来实现序列化时，就可以保证只序列化实际存储的那些元素，而不是整个数组，从而<strong>节省空间和时间</strong>。</p>
<h5 id="2-构造方法分析"><a href="#2-构造方法分析" class="headerlink" title="2.构造方法分析"></a>2.构造方法分析</h5><p>根据initialCapacity 初始化一个空数组，如果值为0，则初始化一个空数组:</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token comment" spellcheck="true">/**
 * 根据initialCapacity 初始化一个空数组
 */</span>
<span class="token keyword">public</span> <span class="token function">ArrayList</span><span class="token punctuation">(</span><span class="token keyword">int</span> initialCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>initialCapacity <span class="token operator">></span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>elementData <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">[</span>initialCapacity<span class="token punctuation">]</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>initialCapacity <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>elementData <span class="token operator">=</span> EMPTY_ELEMENTDATA<span class="token punctuation">;</span>
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
        <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">IllegalArgumentException</span><span class="token punctuation">(</span><span class="token string">"Illegal Capacity: "</span><span class="token operator">+</span>
                                           initialCapacity<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>不带参数初始化，默认容量为10:</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token comment" spellcheck="true">/**
 * 不带参数初始化，默认容量为10
 */</span>
<span class="token keyword">public</span> <span class="token function">ArrayList</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>elementData <span class="token operator">=</span> DEFAULTCAPACITY_EMPTY_ELEMENTDATA<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>通过集合做参数的形式初始化：如果集合为空，则初始化为空数组：</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token comment" spellcheck="true">/**
 * 通过集合做参数的形式初始化
 */</span>
<span class="token keyword">public</span> <span class="token function">ArrayList</span><span class="token punctuation">(</span>Collection<span class="token operator">&lt;</span><span class="token operator">?</span> <span class="token keyword">extends</span> <span class="token class-name">E</span><span class="token operator">></span> c<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    elementData <span class="token operator">=</span> c<span class="token punctuation">.</span><span class="token function">toArray</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment" spellcheck="true">//参数c为实现了Collection的类，toArray为Collection接口定义方法</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>size <span class="token operator">=</span> elementData<span class="token punctuation">.</span>length<span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token comment" spellcheck="true">// c.toArray might (incorrectly) not return Object[] (see 6260652)</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>elementData<span class="token punctuation">.</span><span class="token function">getClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">!=</span> Object<span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token comment" spellcheck="true">//Arrays.copyOf返回类型依赖于第一个参数的类型，此处防止Arrays.copyOf不返回 Object[]类型数据，bug见https://bugs.openjdk.java.net/browse/JDK-6260652</span>
            elementData <span class="token operator">=</span> Arrays<span class="token punctuation">.</span><span class="token function">copyOf</span><span class="token punctuation">(</span>elementData<span class="token punctuation">,</span> size<span class="token punctuation">,</span> Object<span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment" spellcheck="true">//注意此处，仅拷贝实际数据长度</span>
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
        <span class="token comment" spellcheck="true">// replace with empty array.</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>elementData <span class="token operator">=</span> EMPTY_ELEMENTDATA<span class="token punctuation">;</span><span class="token comment" spellcheck="true">//c参数集合长度为0，那么elementData赋值为空的数组对象</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<h5 id="3-主干方法"><a href="#3-主干方法" class="headerlink" title="3.主干方法"></a>3.主干方法</h5><h6 id="trimToSize-方法："><a href="#trimToSize-方法：" class="headerlink" title="trimToSize()方法："></a>trimToSize()方法：</h6><blockquote>
<p>用来最小化实例存储，将容器大小调整为当前元素所占用的容量大小。</p>
</blockquote>
<pre class="line-numbers language-java"><code class="language-java"><span class="token comment" spellcheck="true">/**
 * 这个方法用来最小化实例存储。
 */</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">trimToSize</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    modCount<span class="token operator">++</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>size <span class="token operator">&lt;</span> elementData<span class="token punctuation">.</span>length<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        elementData <span class="token operator">=</span> <span class="token punctuation">(</span>size <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span>
          <span class="token operator">?</span> EMPTY_ELEMENTDATA
          <span class="token operator">:</span> Arrays<span class="token punctuation">.</span><span class="token function">copyOf</span><span class="token punctuation">(</span>elementData<span class="token punctuation">,</span> size<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<hr>
<h6 id="clone方法"><a href="#clone方法" class="headerlink" title="clone方法"></a>clone方法</h6><blockquote>
<p>用来克隆出一个新数组。</p>
</blockquote>
<pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">public</span> Object <span class="token function">clone</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">try</span> <span class="token punctuation">{</span>
        ArrayList<span class="token operator">&lt;</span><span class="token operator">?</span><span class="token operator">></span> v <span class="token operator">=</span> <span class="token punctuation">(</span>ArrayList<span class="token operator">&lt;</span><span class="token operator">?</span><span class="token operator">></span><span class="token punctuation">)</span> <span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">clone</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        v<span class="token punctuation">.</span>elementData <span class="token operator">=</span> Arrays<span class="token punctuation">.</span><span class="token function">copyOf</span><span class="token punctuation">(</span>elementData<span class="token punctuation">,</span> size<span class="token punctuation">)</span><span class="token punctuation">;</span>
        v<span class="token punctuation">.</span>modCount <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
        <span class="token keyword">return</span> v<span class="token punctuation">;</span>
    <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">CloneNotSupportedException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token comment" spellcheck="true">// this shouldn't happen, since we are Cloneable</span>
        <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">InternalError</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>通过调用<code>Object</code>的<code>clone()</code>方法来得到一个新的<code>ArrayList</code>对象，然后将<code>elementData</code>复制给该对象并返回。</p>
<hr>
<h6 id="add-E-e-方法"><a href="#add-E-e-方法" class="headerlink" title="add(E e)方法"></a>add(E e)方法</h6><blockquote>
<p>在数组末尾添加元素</p>
</blockquote>
<pre class="line-numbers language-java"><code class="language-java"><span class="token comment" spellcheck="true">/**
 * 在数组末尾添加元素
 */</span>
<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">add</span><span class="token punctuation">(</span>E e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token function">ensureCapacityInternal</span><span class="token punctuation">(</span>size <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment" spellcheck="true">// Increments modCount!!</span>
    elementData<span class="token punctuation">[</span>size<span class="token operator">++</span><span class="token punctuation">]</span> <span class="token operator">=</span> e<span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>看到它首先调用了<code>ensureCapacityInternal()</code>方法.注意参数是size+1。至于为什么看下去就明白了~</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">ensureCapacityInternal</span><span class="token punctuation">(</span><span class="token keyword">int</span> minCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token function">ensureExplicitCapacity</span><span class="token punctuation">(</span><span class="token function">calculateCapacity</span><span class="token punctuation">(</span>elementData<span class="token punctuation">,</span> minCapacity<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>
<p>这个方法里又嵌套调用了两个方法:计算容量+确保容量</p>
<p>计算容量：如果elementData是空，则返回默认容量10和size+1的最大值，否则返回size+1</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">int</span> <span class="token function">calculateCapacity</span><span class="token punctuation">(</span>Object<span class="token punctuation">[</span><span class="token punctuation">]</span> elementData<span class="token punctuation">,</span> <span class="token keyword">int</span> minCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>elementData <span class="token operator">==</span> DEFAULTCAPACITY_EMPTY_ELEMENTDATA<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> Math<span class="token punctuation">.</span><span class="token function">max</span><span class="token punctuation">(</span>DEFAULT_CAPACITY<span class="token punctuation">,</span> minCapacity<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment" spellcheck="true">//DEFAULT_CAPACITY即为10</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> minCapacity<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>计算完容量后，进行确保容量可用：(modCount不用理它，它用来计算修改次数)</p>
<p>如果<code>size+1 &gt; elementData.length</code>证明数组已经放满，则增加容量，调用<code>grow()</code>。</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">ensureExplicitCapacity</span><span class="token punctuation">(</span><span class="token keyword">int</span> minCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    modCount<span class="token operator">++</span><span class="token punctuation">;</span>

    <span class="token comment" spellcheck="true">// overflow-conscious code</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>minCapacity <span class="token operator">-</span> elementData<span class="token punctuation">.</span>length <span class="token operator">></span> <span class="token number">0</span><span class="token punctuation">)</span>
        <span class="token function">grow</span><span class="token punctuation">(</span>minCapacity<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>增加容量：默认1.5倍扩容。</p>
<ol>
<li>获取当前数组长度=&gt;oldCapacity</li>
<li>oldCapacity&gt;&gt;1 表示将oldCapacity右移一位(位运算)，相当于除2。再加上1，相当于新容量扩容1.5倍。</li>
<li>如果<code>newCapacity&gt;1=1</code>,<code>1&lt;2</code>所以如果不处理该情况，扩容将不能正确完成。</li>
<li>如果新容量比最大值还要大，则将新容量赋值为VM要求最大值。</li>
<li>将elementData拷贝到一个新的容量中。</li>
</ol>
<pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">grow</span><span class="token punctuation">(</span><span class="token keyword">int</span> minCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment" spellcheck="true">// overflow-conscious code</span>
    <span class="token keyword">int</span> oldCapacity <span class="token operator">=</span> elementData<span class="token punctuation">.</span>length<span class="token punctuation">;</span>
    <span class="token keyword">int</span> newCapacity <span class="token operator">=</span> oldCapacity <span class="token operator">+</span> <span class="token punctuation">(</span>oldCapacity <span class="token operator">>></span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>newCapacity <span class="token operator">-</span> minCapacity <span class="token operator">&lt;</span> <span class="token number">0</span><span class="token punctuation">)</span>
        newCapacity <span class="token operator">=</span> minCapacity<span class="token punctuation">;</span><span class="token comment" spellcheck="true">//注意这里要对原长度为0和1进行处理，若不处理，则扩容长度一直为0或1，无法进行下去。</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>newCapacity <span class="token operator">-</span> MAX_ARRAY_SIZE <span class="token operator">></span> <span class="token number">0</span><span class="token punctuation">)</span>
        newCapacity <span class="token operator">=</span> <span class="token function">hugeCapacity</span><span class="token punctuation">(</span>minCapacity<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment" spellcheck="true">// minCapacity is usually close to size, so this is a win:</span>
    elementData <span class="token operator">=</span> Arrays<span class="token punctuation">.</span><span class="token function">copyOf</span><span class="token punctuation">(</span>elementData<span class="token punctuation">,</span> newCapacity<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<hr>
<h6 id="size-1的问题"><a href="#size-1的问题" class="headerlink" title="size+1的问题"></a>size+1的问题</h6><blockquote>
<p>好了，那到这里可以说一下为什么要size+1。</p>
</blockquote>
<p>size+1代表的含义是：</p>
<ol>
<li>如果集合添加元素成功后，集合中的实际元素个数。</li>
<li>为了确保扩容不会出现错误。</li>
</ol>
<p>假如不加一处理，如果默认size是0，则0+0&gt;&gt;1还是0。<br>如果size是1，则1+1&gt;&gt;1还是1。有人问:不是默认容量大小是10吗?事实上，jdk1.8版本以后，ArrayList的扩容放在add()方法中。之前放在构造方法中。我用的是1.8版本，所以默认<code>ArrayList arrayList = new ArrayList();</code>后，size应该是0.所以,size+1对扩容来讲很必要.</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    ArrayList arrayList <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>arrayList<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

输出<span class="token operator">:</span><span class="token number">0</span>
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>事实上上面的代码是证明不了容量大小的，因为size只会在调用<code>add()</code>方法时才会自增。</p>
<hr>
<h5 id="add-int-index-E-element-方法"><a href="#add-int-index-E-element-方法" class="headerlink" title="add(int index, E element)方法"></a>add(int index, E element)方法</h5><pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">,</span> E element<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token function">rangeCheckForAdd</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token function">ensureCapacityInternal</span><span class="token punctuation">(</span>size <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment" spellcheck="true">// Increments modCount!!</span>
    System<span class="token punctuation">.</span><span class="token function">arraycopy</span><span class="token punctuation">(</span>elementData<span class="token punctuation">,</span> index<span class="token punctuation">,</span> elementData<span class="token punctuation">,</span> index <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">,</span>
                     size <span class="token operator">-</span> index<span class="token punctuation">)</span><span class="token punctuation">;</span>
    elementData<span class="token punctuation">[</span>index<span class="token punctuation">]</span> <span class="token operator">=</span> element<span class="token punctuation">;</span>
    size<span class="token operator">++</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>rangeCheckForAdd()是越界异常检测方法。<code>ensureCapacityInternal()</code>之前有讲，着重说一下<code>System.arrayCopy</code>方法：</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">arraycopy</span><span class="token punctuation">(</span>Object src<span class="token punctuation">,</span> <span class="token keyword">int</span> srcPos<span class="token punctuation">,</span> Object dest<span class="token punctuation">,</span> <span class="token keyword">int</span> destPos<span class="token punctuation">,</span> <span class="token keyword">int</span> length<span class="token punctuation">)</span>
<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>
<p>代码解释:</p>
<ul>
<li>Object src : 原数组</li>
<li>int srcPos : 从元数据的起始位置开始</li>
<li>Object dest : 目标数组</li>
<li>int destPos : 目标数组的开始起始位置</li>
<li>int length : 要copy的数组的长度</li>
</ul>
<p>示例：size为6，我们调用<code>add(2,element)</code>方法，则会从index=<code>2+1=3</code>的位置开始，将数组元素替换为从index起始位置为<code>index=2</code>，长度为<code>6-2=4</code>的数据。</p>
<p>异常处理：</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">rangeCheckForAdd</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>index <span class="token operator">></span> size <span class="token operator">||</span> index <span class="token operator">&lt;</span> <span class="token number">0</span><span class="token punctuation">)</span>
        <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">IndexOutOfBoundsException</span><span class="token punctuation">(</span><span class="token function">outOfBoundsMsg</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>
<h5 id="set-int-index-E-element-方法"><a href="#set-int-index-E-element-方法" class="headerlink" title="set(int index,E element)方法"></a>set(int index,E element)方法</h5><pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">public</span> E <span class="token function">set</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">,</span> E element<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token function">rangeCheck</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>

    E oldValue <span class="token operator">=</span> <span class="token function">elementData</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>
    elementData<span class="token punctuation">[</span>index<span class="token punctuation">]</span> <span class="token operator">=</span> element<span class="token punctuation">;</span>
    <span class="token keyword">return</span> oldValue<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

E <span class="token function">elementData</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token punctuation">(</span>E<span class="token punctuation">)</span> elementData<span class="token punctuation">[</span>index<span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>逻辑其实很简单，就是覆盖旧值然后返回。</p>
<h5 id="indexOf-Object-o-方法"><a href="#indexOf-Object-o-方法" class="headerlink" title="indexOf(Object o)方法"></a>indexOf(Object o)方法</h5><blockquote>
<p>根据Object对象获取数组中的索引值。</p>
</blockquote>
<pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">indexOf</span><span class="token punctuation">(</span>Object o<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>o <span class="token operator">==</span> null<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> size<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>elementData<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token operator">==</span>null<span class="token punctuation">)</span>
                <span class="token keyword">return</span> i<span class="token punctuation">;</span>
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> size<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>o<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>elementData<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
                <span class="token keyword">return</span> i<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>如果o为空，则返回数组中第一个为空的索引；不为空也类似。</p>
<p>注意：通过源码可以看到，该方法是允许传空值进来的。</p>
<h5 id="get-int-index-方法"><a href="#get-int-index-方法" class="headerlink" title="get(int index)方法"></a>get(int index)方法</h5><blockquote>
<p>返回指定下标处的元素的值。</p>
</blockquote>
<pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">public</span> E <span class="token function">get</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token function">rangeCheck</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token keyword">return</span> <span class="token function">elementData</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p><code>rangeCheck(index)</code>会检测index值是否合法，如果合法则返回索引对应的值。</p>
<h5 id="remove-int-index-方法"><a href="#remove-int-index-方法" class="headerlink" title="remove(int index)方法"></a>remove(int index)方法</h5><blockquote>
<p>删除指定下标的元素。</p>
</blockquote>
<pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">public</span> E <span class="token function">remove</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment" spellcheck="true">// 检测index是否合法</span>
    <span class="token function">rangeCheck</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment" spellcheck="true">// 数据结构修改次数</span>
    modCount<span class="token operator">++</span><span class="token punctuation">;</span>
    E oldValue <span class="token operator">=</span> <span class="token function">elementData</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment" spellcheck="true">// 记住这个算法</span>
    <span class="token keyword">int</span> numMoved <span class="token operator">=</span> size <span class="token operator">-</span> index <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>numMoved <span class="token operator">></span> <span class="token number">0</span><span class="token punctuation">)</span>
        System<span class="token punctuation">.</span><span class="token function">arraycopy</span><span class="token punctuation">(</span>elementData<span class="token punctuation">,</span> index<span class="token operator">+</span><span class="token number">1</span><span class="token punctuation">,</span> elementData<span class="token punctuation">,</span> index<span class="token punctuation">,</span>numMoved<span class="token punctuation">)</span><span class="token punctuation">;</span>
    elementData<span class="token punctuation">[</span><span class="token operator">--</span>size<span class="token punctuation">]</span> <span class="token operator">=</span> null<span class="token punctuation">;</span> <span class="token comment" spellcheck="true">// clear to let GC do its work</span>

    <span class="token keyword">return</span> oldValue<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<h4 id="ArrayList优缺点"><a href="#ArrayList优缺点" class="headerlink" title="ArrayList优缺点"></a>ArrayList优缺点</h4><p>优点：</p>
<ol>
<li>因为其底层是数组，所以修改和查询效率高。</li>
<li>可自动扩容(1.5倍)。</li>
</ol>
<p>缺点：</p>
<ol>
<li>插入和删除效率不高。</li>
<li>线程不安全。</li>
</ol>
<h4 id="手写ArrayList"><a href="#手写ArrayList" class="headerlink" title="手写ArrayList"></a>手写ArrayList</h4><p>让我们手写的一个简单版的ArrayList吧！</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">MyArrayList</span> <span class="token punctuation">{</span>

    <span class="token comment" spellcheck="true">// 非私有，以简化嵌套类访问</span>
    <span class="token comment" spellcheck="true">// transient 在已经实现序列化的类中，不允许某变量序列化</span>
    <span class="token keyword">transient</span> Object<span class="token punctuation">[</span><span class="token punctuation">]</span> elementData<span class="token punctuation">;</span>

    <span class="token comment" spellcheck="true">//默认容量</span>
    <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> DEFUALT_CAPACITY<span class="token operator">=</span><span class="token number">10</span><span class="token punctuation">;</span>

    <span class="token comment" spellcheck="true">// 用于空实例的 空数组实例</span>
    <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> Object<span class="token punctuation">[</span><span class="token punctuation">]</span> EMPTY_ELEMENTDATA<span class="token operator">=</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

    prvate <span class="token keyword">static</span> <span class="token keyword">final</span> Object<span class="token punctuation">[</span><span class="token punctuation">]</span> DEFAULTCAPACITY_EMPTY_ELEMENTDATA<span class="token operator">=</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

    <span class="token comment" spellcheck="true">// 实际ArrayList集合大小</span>
    <span class="token keyword">private</span> <span class="token keyword">int</span> size<span class="token punctuation">;</span>

     <span class="token comment" spellcheck="true">/**
     * 构造方法
     */</span>
    <span class="token keyword">public</span> <span class="token function">MyArrayList</span><span class="token punctuation">(</span><span class="token keyword">int</span> initialCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>initialCapacity <span class="token operator">></span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">this</span><span class="token punctuation">.</span>elementData <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">[</span>initialCapacity<span class="token punctuation">]</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>initialCapacity <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">this</span><span class="token punctuation">.</span>elementData <span class="token operator">=</span> EMPTY_ELEMENTDATA<span class="token punctuation">;</span>
        <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
            <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">IllegalArgumentException</span><span class="token punctuation">(</span><span class="token string">"Illegal Capacity: "</span><span class="token operator">+</span>
                    initialCapacity<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">public</span> <span class="token function">MyArrayList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">this</span><span class="token punctuation">(</span>DEFAULT_CAPACITY<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">add</span><span class="token punctuation">(</span>Object o<span class="token punctuation">)</span>
    <span class="token punctuation">{</span>
        <span class="token comment" spellcheck="true">//1. 判断数据容量是否大于 elementData</span>
        <span class="token function">ensureExplicitCapacity</span><span class="token punctuation">(</span>size<span class="token operator">+</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment" spellcheck="true">//2. 使用下标进行赋值</span>
        elementData<span class="token punctuation">[</span>size<span class="token operator">++</span><span class="token punctuation">]</span> <span class="token operator">=</span> o<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

      <span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">ensureExplicitCapacity</span><span class="token punctuation">(</span><span class="token keyword">int</span> minCapacity<span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>size <span class="token operator">==</span> elementData<span class="token punctuation">.</span>length<span class="token punctuation">)</span><span class="token punctuation">{</span>
              <span class="token comment" spellcheck="true">// 需要扩容,扩容1.5倍(ArrayList默认扩容1.5倍)</span>
            <span class="token comment" spellcheck="true">// 注意：如果oldCapacity值为1</span>
            <span class="token keyword">int</span> oldCapacity <span class="token operator">=</span> elementData<span class="token punctuation">.</span>length<span class="token punctuation">;</span>
            <span class="token keyword">int</span> newCapacity <span class="token operator">=</span> oldCapacity <span class="token operator">+</span> <span class="token punctuation">(</span>oldCapacity <span class="token operator">>></span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span> 
               <span class="token keyword">if</span> <span class="token punctuation">(</span>newCapacity <span class="token operator">-</span> minCapacity <span class="token operator">&lt;</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
                newCapacity <span class="token operator">=</span> minCapacity<span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
              <span class="token comment" spellcheck="true">// 创建新数组</span>
            Object<span class="token punctuation">[</span><span class="token punctuation">]</span> objects <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">[</span>newCapacity<span class="token punctuation">]</span><span class="token punctuation">;</span>
            <span class="token comment" spellcheck="true">// 将数据复制给新数组</span>
            System<span class="token punctuation">.</span><span class="token function">arraycopy</span><span class="token punctuation">(</span>elementData<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> objects<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> elementData<span class="token punctuation">.</span>length<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token comment" spellcheck="true">// 修改引用</span>
            elementData <span class="token operator">=</span> objects<span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
      <span class="token punctuation">}</span>
        <span class="token keyword">public</span> Object <span class="token function">get</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token function">rangeCheck</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">return</span> elementData<span class="token punctuation">[</span>index<span class="token punctuation">]</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
      <span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">rangeCheck</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>index <span class="token operator">>=</span> size<span class="token punctuation">)</span>
            <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">IndexOutOfBoundsException</span><span class="token punctuation">(</span><span class="token string">"下标越界"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
       <span class="token comment" spellcheck="true">/**
     * 通过下标删除
     * @param index
     * @return
     */</span>
      <span class="token keyword">public</span> Object <span class="token function">remove</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">{</span>
           <span class="token function">rangeCheck</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>
        Object oldValue <span class="token operator">=</span> elementData<span class="token punctuation">[</span>index<span class="token punctuation">]</span><span class="token punctuation">;</span>
        <span class="token comment" spellcheck="true">// 找出置换结束位置</span>
        <span class="token keyword">int</span> numMoved <span class="token operator">=</span> size <span class="token operator">-</span> index <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">;</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>numMoved <span class="token operator">></span> <span class="token number">0</span><span class="token punctuation">)</span>
            <span class="token comment" spellcheck="true">// 从 index+1 开始 将值覆盖为 index-numMoved 的值</span>
            System<span class="token punctuation">.</span><span class="token function">arraycopy</span><span class="token punctuation">(</span>elementData<span class="token punctuation">,</span> index<span class="token operator">+</span><span class="token number">1</span><span class="token punctuation">,</span> elementData<span class="token punctuation">,</span> index<span class="token punctuation">,</span> numMoved<span class="token punctuation">)</span><span class="token punctuation">;</span>
        elementData<span class="token punctuation">[</span><span class="token operator">--</span>size<span class="token punctuation">]</span> <span class="token operator">=</span> null<span class="token punctuation">;</span> <span class="token comment" spellcheck="true">// clear to let GC do its work</span>
               <span class="token keyword">return</span> oldValue<span class="token punctuation">;</span>
      <span class="token punctuation">}</span>

      <span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">remove</span><span class="token punctuation">(</span>Object o<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> index <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> index <span class="token operator">&lt;</span> size<span class="token punctuation">;</span> index<span class="token operator">++</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
                <span class="token keyword">if</span> <span class="token punctuation">(</span>o<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>elementData<span class="token punctuation">[</span>index<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                    <span class="token function">remove</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>
                    <span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
                <span class="token punctuation">}</span>
            <span class="token punctuation">}</span>
        <span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

<span class="token punctuation">}</span>
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